exhaled gas concentrations can also provide information on the uniformity of inspired gas distribution throughout the lungs when we exhale the concentration of gases in the expert changes from beginning to end of expiration if a gas is distributed non uniformly throughout the lungs the changes in concentration of the gas throughout expiration will be greater than normal the single breath nitrogen washout test is based on this hypothesis the seated patient takes two breaths of air and exhales to residual volume then the valve on the mouthpiece is turned to administer 100 percent oxygen the patient takes a deep breath of the oxygen to total lung capacity and then exhale slowly to residual volume the concentration of nitrogen in the expired gas is measured continuously using a nitrogen meter additionally exhale volume is measured using a pneumo tachograph a plot of exhaled nitrogen concentration versus exhaled volume is generated a typical plot appears as follows for relatively discrete phases of the curve one two three and four are depicted phase one represents the initial portion of the expert which is devoid of nitrogen since pure oxygen was inhaled during the previous breath this portion represents gas residing in the proximal airways from the end of the prior inspiration Phase two constitutes a mixture of gas from the central Airways and the alveoli phase three is defined by a mixture of gases from the lung apices mid-long zones and lung bases and phase four constitutes gas arising predominantly from the apices as the Airways at the base is closed as lower lung volumes are approached with inspiration most inspired gas goes to the bases where there are more alveoli and where the alveoli are less distended that those in the apices and hence can expand more as a result the inhaled pure oxygen is primarily distributed to the bases diluting the nitrogen in that region and accounting for a concentration difference in nitrogen between the apices and the bases during exhalation the decreased elastic recoil of the lung and narrowing of small Airways caused closure of small Airways at the bases of the lungs as this happens the contribution of gas from basilar alveoli to the expired gas concentration decreases and as exhalation progresses more exhaled gas comes from the apices which represent the less diluted alveolar air which is higher in nitrogen concentration the exhaled volume from the onset of phase four to the end of the expert ory maneuver is known as closing volume remember this is the portion of the exhaled air after the closure of the small Airways so a change in this value reflects changes in the small Airways normal closing volume is about 10% of vital capacity with small Airways disease aging premature closure of Airways or cigarette smoking closing volume is increased in patients greater than 50 years of age closing volume may be 25% of vital capacity so I hope you like this video absolutely make sure to check out the course this video was taken from and to register for a free trial account which will give you access to selected chapters of the course if you want to learn how mat mastery can help you become a great clinician make sure to watch the about my mastery video so thanks for watching and I hope to see you again soon